As we planned for Christmas this year, my partner suggested that maybe we should replace the lights for the tree. “While we’re at it,” she asked, “is there anything out there that’s more controllable than just the ones that twinkle on and off?”

“Well,” I said, “there are NeoPixels…”

We decided that it would be great to have a tree where the lights looked and acted like candles, flickering gently in a soft, warm glow, and fading from a bright orange down to a low reddish glow by the end of the evening. We also wanted to realize the cheesy quote from It’s A Wonderful Life that “every time a bell rings, an angel gets its wings.”

Recently, Federico Fissore added node.js to the package repository for the Arduino Yún. Here’s how you get node to communicate with the Arduino processor on the Yún via the Bridge library.

To do this, you’ll need an Arduino Yún, a microSD card, a microUSB cable and a wifi connection. You should be familiar with the basics of the Arduino Yún and node.js in order to get the most out of this post.

Recently, a colleague of mine was working on a project with an Arduino Yún that involved reading a lot of sensor data using the Yún’s Arduino processor and writing it to the microSD card via the board’s Linux processor to be served to other devices via HTTP. She found that it took anywhere from 20 milliseconds to several seconds to get the data from the sensors written to the SD card. “Why is it not real-time?” she asked me.

Welcome to the world of embedded operating systems. They are not realtime. “Realtime” can mean many things. In automotive systems, for example, your car’s braking system had better react in “realtime” or you’re dead. That might mean only a couple of milliseconds. When measuring high-speed rotation, it might even mean microseconds.

My colleague was measuring her function’s response time in tens to hundreds of milliseconds. That function read one controller’s analog input pin, sent the result via asynchronous serial to another controller, and then stored the result on an SD card. I haven’t measured it, but I’d wager you’ll see the same response times on a BeagleBone or Raspberry Pi, or any embedded Linux board. Here’s why:

All computers run programs. In a microcontroller like the Arduino, there’s only one program running. That program is made up of instructions stored in memory, in a particular order. The computer moves through those instructions one at a time. Sometimes it jumps around in the

At the electrical level, all computers are made up of transistors, so in computing, the fastest version of “realtime” means “how fast can you read and act on a changing voltage on a transistor?” Some of the input/output pins of a microcontroller are usually hardware interrupt pins, meaning that they can be programmed such that if there’s a change on that pin, the program running on the controller is immediately interrupted and a special function is run. This function, called an interrupt service routine or ISR, is typically very short. Normally an ISR simply stores the result of the interrupt in a variable. Then the processor returns to the main program.

Operating systems, both on small boards like this and on servers and personal computers, do not guarantee a minimum response time to physical I/O. They are written to optimize processor sharing between programs, so the scheduler, a core part of the operating system, doles out processor time to each program. Even before the programs you run get time, there are OS tasks that take time. Disk writing is one of the most time-intensive tasks. Perhaps the only longer task is a network transaction, because the data is going through many computers, each with its own operating system.

In systems that have to have real-time response, you typically use one of two options:

1) no operating system. Your processor runs one program only. Like Arduino

2) a “realtime operating system” or RTOS. RTOSes are stripped-down operating systems that leave many core functions out. As of yet, all RTOSes are custom packages, not very user friendly, though there has been some work lately on realtime linux. I haven’t seen one running on any of the hobbyist boards, but i wouldn’t be surprised if we don’t see one in the next year or so.

When we designed the Yun, we decided we’d give users the benefits of both real-time and an operating system. What you do on the 32U4 (the Arduino processor) is real-time because there is no operating system. What you do on the linino side is not, because it’s running linux, an operating system. The typical approach to a networked project (whether a Yun or other system) is to do all the real-time operations on a non-OS micro controller, then send the results to an operating system computer in non-real-time, after the action has happened.

Here’s a use case that illustrates the use of real-time and an operating system together:

Let’s say you’re using a rotary encoder to measure the speed of a wheel on a remote control vehicle. You want to display the speed on a dashboard screen that’s networked to the vehicle over Wifi.

Rotary encoders measure rotation by counting pulses generated by a rotating shaft. When the rotation is fast, the pulses happen VERY fast, and you need real-time to catch them. On a micro controller, this is typically sensed using hardware interrupts. These are sub-circuits of the micro controller that can interrupt the program flow when a change happens on a given I/O pin. They force the program to jump to a function to handle the interrupt. You usually want that function to be very short — typically all it does it to increment or decrement a variable counting the changes. The Arduino Uno has two hardware interrupts, and Paul Stoffregen’s encoder library allows you to use them for reading encoders.

The way you’d build this app is to have the rotary encoder attached to the hardware interrupts of a micro controller. This controller is your physical I/O controller. You’d write a program for the micro controller that calculates the speed based on the pulse rate and sends that serially to another controller connected to a display. One function counts the pulses. That’d be done by the function called by the interrupt (these are called Interrupt service routines, or ISRs). Another function calculates a running speed based on the changing count. A third function might control the movement of the vehicle’s steering based on the value. A fourth function sends the calculated value over a serial connection to the display computer.

The serial connection of the physical controller be connected to a networked modem like the Wifi shield or a Bluetooth radio, but that modem is just another single-function controller. That transmission takes time, and you don’t want to take processor time away from counting the pulses, so your physical I/O controller doesn’t handle this transmission, it only counts pulses and sends the value on. The radio controller handles the network connection. It transmits the data on to a display computer or a server. That server is typically running an operating system, and not working in real time, but that’s okay, because humans aren’t going to react in more than a half-second or so most of the time. What you see onscreen is typically a report of the sensor readings, but it’s an average or aggregated reading, not the raw, realtime reading. The delay depends on the transmission time of the data. Perhaps you have a virtual steering wheel onscreen that then directs the card, but this is not real-time either. The user gets to steer the car, but what she’s really doing is affecting the gross movement of the steering, not the fine control over the axle. She’s changing the overall balance, but the physical I/o controller is the only part acting in real-time to the sensors.

It is possible for a controller that’s running an operating system to have hardware interrupts, and for it to have interrupt service routines. But those are typically part of the operating system kernel, not part of the user programs. You have to write custom kernel routines and restart the OS in order to use them. So unless you’re a kernel programmer, you typically won’t use these on BeagleBone, Raspberry Pi, or Arduino Yun. This is why there are many projects that combine the Pi or the Bone with an Arduino or other non-OS controller to do real-time sensing, and it’s why the Yun has a separate processor for that as well.

A typical personal computer is made up of several controllers, in fact. There’s normally a CPU, a network controller, a controller inside any disk drive (if it’s a drive with a motor), a graphics processor, a hardware bus controller, and more. These are usually tied together with synchronous serial communication like SPI or I2C or some other means of communication. The dedicated controllers handle their subsystems in realtime, and report back to the CPU, which reports in user time (not real time).

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Blinkinlabs is shipping their product now after their successful KickStarter campaign, and I got mine in the mail a couple days ago. I had a few minutes and thought I’d get a basic example up and running. The Blinkinlabs documentation was confusing to me, however. The “Simplest BlinkyTape Sketch” wasn’t. The code was more complex than I had hoped, and I couldn’t find documentation of the API, but I noticed there were some examples that used the Adafruit NeoPixel library. Those examples didn’t run for me, but I figured if there were NeoPixel examples in the BlinkyTape repository, it should be possible to run the thing with that library.

A quick look at the schematics for BlinkyTape confirmed that it was indeed running the Atmega32U4, same as the Arduino Leonardo (their documentation’s good on setting the tape up as a Leonardo). Furthermore, the data in of the lights is connected to pin 13. From there, it was simply a matter of modifying the Adafruit NeoPixels to work on pin 13.

Like the BlinkyTape examples, most of the Adafruit NeoPixel library examples were all about color scrolling. I dislike color scrolling. Call me old and cranky, but I remember a time when lighting designers made color choices and stood by them; when the designers of lighting tools gave us controls that let us choose those colors easily. I’m hoping that someday we’ll look back on this era see color scrolling as a lighting design choice the same way as we see the mullet in hairstyle choice: a bad idea we had to go through to get to something more pleasant. Now you kids, get outta my yard! </cranky old man rant>

My sketch works like this: you send a serial string to the Arduino starting with a capital C, followed by the pixel number, the red level (0-255), the green level (0-255) and the blue level (0-255). The NeoPixel library then sets the color of the pixel you chose. That’s it.

Now that I know this thing’s so simple to use, I can see a lot of projects with it. Love the built-in processor too, makes it even more convenient. Makes me want to order a whole lot more NeoPixels from Adafruit too.

I know how to send data from Arduino to (Processing, OpenFrameworks, etc). But how to I send data from (Processing, OpenFrameworks, etc) to Arduino?

Many people don’t seem to know that Arduino has built-in functions for parsing streams of data. Back with version 1.0.1 Michael Margolis’ excellent TextFinder library was merged into the core Stream library. So you can send an ASCII string like this:

123, 456, 789

And Arduino can read it and convert the numeric characters back into numeric values.

For example, let’s say you want to send a string to set the values of two LEDs that you’re going to fade using the analogWrite() command. Your data might look like this:

P1, 255\n
P2, 127\n

(the \n represents a newline character, ASCII 10)

Using the parsing functions, you can look for the initial P character, then parse the numeric string that follows it until a non-numeric character (like the comma) comes along. Then you can do another parse until the next non-numeric character (like the newline at the end of the line) comes along. It’s as simple as this:

So the next time you’re trying to figure out how to read data in Arduino, first decide what your data looks like, then get to know the Stream functions. In addition to find() and parseInt(), there’s also readBytes(), readBytesUntil(), parseInt(), parsefloat(), and a few other goodies.

The really nice thing about these functions is that they work on any library based on Stream. So, for example, you can also use them on the Ethernet and WiFi and GSM Client and Server classes. This makes parsing network data much simpler.

Let’s say you’re a fan of one of the bands in The Deli magazine’s new bands poll. There are so many good bands there, like Teen Girl Scientist Monthly, and others. And let’s say you were looking for a project for an Arduino WiFi shield or Ethernet shield. Why not make your Arduino vote for your favorite band? It’s not hard to do.

First you need to know how to scrape the webpage for the URL of the vote. If you view source on the page, you’ll see it nice and clear:

And give it a go! Remember, no cheating. You can only vote once per IP address. But if you know a friend with several WiFi shields or Ethernet shields, well, that’s only one IP address per shield, right?

I get a lot of people asking me how to send tweets from an Arduino through an Ethernet or WiFi shield. It turns out to be a bit tricky, because Twitter uses OAuth to manage authentication, which is difficult to program on an Arduino due to its limited program memory size. Most libraries that send tweets from an Arduino do so by handling the authentication through another site. There are a couple of libraries that attempt to do it directly from the Arduino, but I haven’t seen one I like yet that sticks to the Arduino API style and/or doesn’t take up a lot of memory (though I am ever hopeful someone will write one). Since not everyone knows how to write a server-side middleware program to handle the authentication, I decided to see if I could get an existing service to take care of it for me.

My first thought was to use If This Then That, but I don’t know the details of their API well enough, so I went with something I already knew: Pachube. Yes I know they have a newer name, but due to some complications I’m not sure are resolved, I’m using the old name. Here’s how to do it:

The address you type into your browser’s address bar is often the location of a particular document on a server. For example, http://tigoe.net/index.html refers to an HTML document living in the main directory of my server, tigoe.net. But URLs can be used to represent more than a file address. They can also be used to set or get the state of web-based application. For example, http://www.mystore.com/item/3045/price could be used to tell the application you want the price of item 3045. If you want the set the price, you could use http://www.mystore.com/item/3045/price/4.99. Web frameworks like Sinatra (for Ruby), Flask (for Python) and Express (for JavaScript through node.js) make it possible for you to build a web server that uses Representational State Transfer, or REST, as the control protocol for your application.

REST is a style of formatting URLs such that the URL itself describes the state of the thing it’s addressing. In the store example above, the URL is a representation of the item in the store (http://www.mystore.com/item/3045/price). To change the state of the item, you use another URL to represent that change (http://www.mystore.com/item/3045/price/4.99). At its simplest, REST means organizing your web application so that the URLs provide clear, sensible meaning as to what’s going on. For a more detailed explanation, see Understanding REST or Building Web Services the REST way. In this post, you’ll learn how to use a RESTian scheme as a communications protocol between a microcontroller and a web page using node.js and a little client-side JavaScript in the middle.

Node.js is great for making web services, and node-serialport makes it very easy to connect to serial devices on your computer. In my last post, I showed how to connect an Arduino microcontroller application to a web page using Node. This post expands on that, introducing how to use JavaScript Object Notation (JSON) from Arduino all the way through to your HTML page.

The beauty of node is that it’s JavaScript, so you get to use everything Javacript gives you, including its wonderful lightweight data format, JSON, or JavaScript Object Notation. JSON describes data objects using arrays of key-value pairs separated by colons. If you want to add more properties to an object, just add another array element. the value of an element can be a JSON object itself. When you’ve got an application that uses JavaScript on the server side and on the client side, it makes sense to use JSON to describe your data all the way through, so you can just pass it around without a lot of conversion.

Arduino doesn’t speak JSON natively. There are a couple JSON parser libraries out there for Arduino, but I haven’t seen one that I like yet. All of the ones I’ve seen expect more pointer knowledge from the user than I’d like. So for this example you’ll assemble your own JSON string using Arduino’s String class, and letting node.js turn it into a JSON object.